Characterization of near field transducers

ABSTRACT

An approach for characterizing an optical near field transducer (NFT) involves providing excitation radiation to the NFT. The NFT emits photoluminescent radiation in response to the excitation radiation. The output radiation from the NFT is filtered so that a portion of the photoluminescent radiation emitted by the NFT passes through the filter and the excitation radiation is substantially blocked. A detector detects the portion of photoluminescent radiation and outputs an electrical signal in response to detection of the portion of photoluminescent radiation.

RELATED PATENT DOCUMENTS

This application claims the benefit of provisional Patent ApplicationSer. No. 61/637,507 filed on Apr. 24, 2012, to which priority is claimedpursuant to 35 U.S.C. §119(e) and which is hereby incorporated herein byreference in its entirety.

SUMMARY

Embodiments of the disclosure are directed to approaches forcharacterizing near field transducers. Some embodiments are directed toa system that includes an excitation light source configured to provideexcitation radiation to a near field transducer (NFT) subassemblyincluding an NFT optical antenna. The system comprises an optical filterconfigured to substantially pass a portion of photoluminescent radiationemitted by the NFT optical antenna in response to the excitationradiation and to substantially block the excitation radiation. Adetector is configured to detect the portion of photoluminescentradiation and to output an output signal in response to detection of theportion of photoluminescent radiation.

Some embodiments are directed to a method for characterizing NFTsubassemblies. Excitation radiation is provided to a near fieldtransducer (NFT) subassembly. The output radiation from the NFTsubassembly is filtered. Filtering the output radiation comprisespassing a portion of photoluminescent radiation emitted by the NFTsubassembly in response to the excitation radiation and substantiallyblocking the excitation radiation transmitted by the NFT subassembly.The portion of photoluminescent radiation is detected and an outputsignal is generated in response to the detecting.

These and other features and aspects of various embodiments may beunderstood in view of the following detailed discussion and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a process flow diagram that illustrates a high-levelsummary of a portion of a process for fabricating a slider that mayinclude an near field transducer (NFT) characterization step accordingto the approaches discussed herein;

FIG. 2 provides a cross-sectional diagram that illustrates an NFTsubassembly that can be characterized using the approaches discussedherein;

FIG. 3 depicts a system for characterizing NFT subassemblies inaccordance with some embodiments;

FIG. 4A provides an example spectral characteristic of the excitationradiation for the systems of FIGS. 3 and 5;

FIG. 4B provides an example spectral characteristic of photoluminescentradiation emitted by an NFT in response to the excitation radiation;

FIG. 4C illustrates shortwave pass filtered radiation that is used tocharacterize the NFT under test in accordance with various embodiments.

FIG. 5 is a block diagram of a system for characterizing NFTsubassemblies according to various embodiments;

FIG. 6 shows a possible spectral characteristic for the shortwave passfilter used in the system of FIG. 5;

FIG. 7 shows a possible spectral characteristic of a suitable dichroicbeam splitter used in the system of FIG. 5;

FIG. 8 is a flow diagram of a method to characterize an NFT subassemblyin accordance with some embodiments;

FIGS. 9A-9D show optical images of the transmitted light from agap-plasmon NFT acquired using the system shown in FIG. 5;

FIG. 10 shows a log-log plot of photoluminescence vs. monitor power,obtained using the NFT characterization system described in FIG. 5 fordevices that have a lollipop NFT; and

FIG. 11 shows the variation in luminescence with for lollipop NFTsmeasured using the NFT characterization system shown in FIG. 5.

DETAILED DESCRIPTION

The present disclosure relates to characterization of optical componentsused in applications such as heat assisted magnetic recording (HAMR). AHAMR device utilizes a magnetic recording media (e.g., hard disk) thatis able to overcome superparamagnetic effects that limit the areal datadensity of typical magnetic media. In order to record on this media, asmall portion of the media is locally heated while being written to by amagnetic write head. A coherent light source such as a laser can providethe energy to create these hot spots, and optical components, e.g.,built in to a slider that houses the write head, are configured directthis energy onto the media.

When applying light to a HAMR medium, light from the light source isconcentrated into a small hotspot over the track where writing is takingplace. As a result of what is known as the diffraction limit, opticalcomponents cannot be used to focus light to a dimension that is lessthan about half the wavelength of the light. For example, the lasersused in some HAMR designs produce light with wavelengths on the order of800-900 nm, yet the desired hot spot is on the order of 50 nm or less.Thus the desired hot spot size is well below half the wavelength of thelight, and, due to diffraction, optical focusers cannot be used toobtain the desired hot spot size. As a result, an optical near fieldtransducer (NFT) is employed to create these small hotspots on themedia.

The NFT is a near-field optics device designed to reach local surfaceplasmon conditions at a designed wavelength. Example NFT transducers mayinclude a plasmonic optical antenna or a metallic aperture and afocusing element. The focusing element concentrates light on thetransducer region (e.g., at the focal region) near where the opticalantenna or a metallic aperture is located. Example focusing elements mayinclude solid immersion lenses (SIL), solid immersion mirrors (SIM),and/or three-dimensional channel waveguide for light delivery to a NFT.The NFT is designed to achieve surface plasmon resonance in response tothis concentration of light.

Surface plasmons (SPs) are collective oscillations of surface chargesthat are confined to an interface between a dielectric and a metal. WhenSPs are resonantly excited by an external optical field, the fieldamplitude in the vicinity of the surface may be orders of magnitudegreater than that of the incident field. Moreover, the region ofenhanced field may be tightly confined to a spot much smaller than theincident wavelength. At resonance, a high electric field surrounds theNFT due to the collective oscillations of electrons at the metalsurface. Part of this field will tunnel into a storage medium and getabsorbed, thereby raising the temperature of a spot on the media as itbeing recorded.

The NFT may be located near an air bearing surface (ABS) of a sliderused for magnetic data storage, and may be placed in close proximity toa write head that is also part of the slider. This co-location of theNFT with the write head facilitates heating the hot spot during writeoperations. The focusing element, e.g., waveguide, and NFT may be formedas an integral part of the slider that houses the write head. Otheroptical elements, such as couplers, mirrors, prisms, etc., may also beformed integral to the slider. The optical elements used in HAMRrecording heads are generally referred to as integrated optics devices.

The field of integrated optics relates to the construction of opticsdevices on substrates, sometimes in combination with electroniccomponents, to produce functional systems or subsystems. For example, anintegrated optics device may be built up on a substrate using layerdeposition techniques. In reference now to FIG. 1, a process flowdiagram illustrates a high-level, short summary of a portion of aprocess for fabricating a slider that includes integrated opticsincluding an NFT and various optical coupling and/or light positioningelements. Block 102 represents a wafer-level stage. A wafer 104 isformed using semiconductor manufacturing processes (e.g., thin filmdeposition, chemical-mechanical polishing/planarization, etc.) and eachwafer 104 generally includes a plurality of sliders (e.g., slider 106)that are later cut into bars for further processing. Each slider 106includes an NFT subassembly comprising a waveguide focusing element andan NFT.

Block 112 represents an upstream stage where the wafer 104 has been cutinto bars 114. Each bar 114 includes a plurality of sliders that arebatch-processed. Stage 112 may involve attaching top bond pads (e.g.,part of a slider-gimbal electrical interface). At the bar stage shown inBlock 112, it can facilitate the manufacturing process to characterizethe NFT subassemblies in the sliders 106 prior to proceeding withattachment of the lasers and subsequent manufacturing processing stepsto complete the slider fabrication.

Block 122 represents a bar-level laser attach stage. Stage 122 mayinvolve removing sacrificial cavity fill material, attaching the lasersand/or aligning the lasers with the NFT subassemblies. The lasers (e.g.laser diode 125) may be placed on the bars using a pick-and-placemachine 124, and thereafter bonded to the slider (e.g., slider 106) viaa reflow operation (e.g., application of heat to melt the solder bumps)to form assembly 128. Block 132 represents a stage for forming ahead-gimbal assembly (HGA). Additional optical, electrical and/ormagnetic tests may be performed on the completed head-gimbal assembly134. In some cases, the manufacturing process may be facilitated bytesting the NFT subassemblies at the bar stage before proceeding withthe laser attachment and subsequent manufacturing steps.

FIG. 2 provides a cross-sectional diagram that illustrates an NFTsubassembly 200 that can be characterized using the approaches discussedherein. The NFT subassembly 200 shown in FIG. 2 is fabricated in aslider 205 and includes input waveguide coupler 209, first mirror 211,second mirror 212, solid immersion mirror 202, and NFT optical antenna203. NFT subassemblies 200 that operate by end-fire technique as shownin FIG. 2 may in incorporated into each slider 106 of a bar 114 shown inFIG. 1. The NFT subassembly 200 illustrates one particularconfiguration, although it will be appreciated that many configurationsof NFT subassemblies are possible and can be characterized by theapproaches discussed herein.

In the example illustrated in FIG. 2, the NFT subassembly 200 receiveslight emitted from a light source 290 via an input waveguide coupler 209which is a three-dimensional channel waveguide of finite wide waveguidecore. In normal use, the light source 290 may comprise an edge emittingor surface emitting laser diode, for example. The light emerging fromthe waveguide coupler 209 is directed in a solid immersion mirror (SIM),or planar solid immersion mirror (PSIM) indicated by way of SIM sidewall202 in FIG. 2, by a first mirror 211 and a second mirror 212. An opticalantenna NFT 203 is located at the focus point of the SIM 202.

The optical antenna NFT 203 shown in FIG. 2 comprises a “lollipop”configuration that combines a circular disc with a peg, although otherconfigurations may be used. The lollipop dimensions are selected tofunction as an antenna for the incident light, to resonate at theexcitation wavelength, and to transfer energy into the peg and thus tothe medium via the feedgap at the tip 203 a of the NFT optical antenna203. The NFT optical antenna 203 (also referred to as an NFT) is atransducer that can be made of any known plasmonic material (e.g., Au,Ag, Cu, ZrN) and may be positioned at or near the focal region of thelight 210.

The waveguide core 201 may be formed from any material that has a higherindex of refraction than cladding. For example, the waveguide core 201may be made from Ta₂O₅, TiO₂, ZnS, SiN. The PSIM 202 may be formed as aparabolic cutout of surrounding dielectric waveguide material (e.g.,Al₂O₃, SiO₂, SiOxNy, MgO, HfO₂, Y₂O₃, Ta₂O₅, TiOx). The cutout may beformed from/coated with a reflective material (e.g., Au, Al), so thatlight rays 210 entering the PSIM 202 by way of waveguide core 201 arefocused to a focal region to strongly couple to the NFT optical antenna203 and generate surface plasmons.

As previously discussed in connection with FIG. 1, the manufacturingprocess can be facilitated by characterization of the optical componentsof a slider, including the NFT subassembly, such as NFT subassembly 200.For example, NFT subassemblies can be tested at the wafer stage, the barstage, or even individually prior to laser placement.

Dark field microscopy has been attempted to characterize opticalantennas by measuring the light scattering from NFT, however thischaracterization technique is not suitable for in the presence of anincident beam (“bright field”) in actual devices. Characterization ofthe NFT by the thermo-reflectance pump/probe method measures opticalchanges due to absorption of the NFT, however, the pump/probe method cansuffer from variation due to thermal environment. Some characterizationmethods are be insensitive to certain parameters that are useful totrack in a manufacturing environment.

According to embodiments discussed herein, characterization of the NFTsubassemblies may be accomplished by sensing filtered photoluminescentradiation emitted by the NFT in response to high energy excitationradiation. The photoluminescent radiation is strongly enhanced by thelocal surface plasmons that are generated at the NFT surface. Thephotoluminescent radiation generated in the NFT includes wavelengthsshorter than the excitation radiation by two-photon excitation.Two-photon luminescence is luminescence excited by two-photonabsorption. Two-photon induced photoluminescence in noble metals such asgold and silver is generally considered as a three-step process.Electrons from occupied d bands are first excited by two-photonabsorption to unoccupied states of the sp-conduction band. Second,subsequent intraband scattering processes move the electrons closer tothe Fermi level. Third, the relaxation of the electron-hole pairrecombines either through nonradiative processes or by emission ofluminescence. The emission of luminescence is proportional to E⁴, whereE denotes the electric-field amplitude. Local surface plasmons at thesurface of the NFT antenna enhance the luminescence significantly.

In various configurations, the characterization system includesshortwave pass spectral filters, notch filters and/or beam splitterswith a wavelength edge that are used to separate the bright field light(e.g., the excitation light) from the photoluminescent light emanatingfrom the NFT.

A system for characterizing NFT subassemblies in accordance with someembodiments is described with reference to FIGS. 3 and 4A-4C. A laser301 emits excitation radiation 303 that passes through a focusing lens305 and illuminates one of the NFT subassemblies 311 disposed on bar314. FIG. 4 provides an exemplary spectral distribution of the focusedexcitation radiation 306 that is centered at wavelength λ_(E). Thespectral distribution diagrams of FIGS. 4A-C are idealized as Gaussiandistributions of arbitrary peak magnitudes, however, it will beappreciated that, in general, the distributions need not be Gaussian. Inresponse to the excitation radiation 306, the NFT subassembly 311 emitswhite-light super-continuum photoluminescence 313 at the feedgap and tipof the optical antenna (see, 203 a, FIG. 2). The NFT subassembly alsotransmits the portion of the excitation radiation that is not absorbedin the NFT subassembly 311.

An exemplary spectral distribution of the electromagnetic radiation 312emerging from the NFT subassembly 311 that includes both aphotoluminescent radiation component 313 and an excitation radiationcomponent 306, is shown in FIG. 4B. In this example, thephotoluminescent radiation component 313 is shown as having an arbitrarypeak or central wavelength, λ_(L), and the excitation radiationcomponent 306 is shown as having an arbitrary peak or centralwavelength, λ_(E). Although the spectral distributions and magnitudes ofFIGS. 4A-4C do not necessarily correspond to actual spectraldistributions and magnitudes of the photoluminescent and excitationradiation, FIG. 4C illustrates that the photoluminescent radiation 313emitted by the NFT includes shorter wavelength radiation and/or hasshorter peak or central wavelength when compared to the excitationradiation 306.

The radiation 312 that emerges from the NFT 311 is collected andcollimated by a lens 315 and passes through a shortwave pass filter 320having a cutoff wavelength, λ_(F). The shortwave pass filter 320substantially removes components of the radiation 312 having awavelength longer than λ_(F). As such, the shortwave pass filter 320substantially absorbs or blocks the excitation radiation component 306and also absorbs or blocks that portion of the photoluminescentradiation that has wavelength greater than λ_(F). The shortwave passfilter substantially passes wavelengths of the photoluminescentradiation with wavelengths greater than λ_(F), including radiation 325shown in FIG. 4C.

Returning now to FIG. 3, the filtered radiation 325 impinges on adetector 330, such as a photomultiplier tube (PMT). The PMT provides anelectrical signal output 335 in response to the incident filteredradiation 325 that can be used to measure the filtered photoluminescentradiation emitted by the NFT subassembly.

FIG. 5 shows another embodiment of a system 500 for characterizing theNFTs by detecting photoluminescent light emanating from the NFTs inresponse to excitation radiation. For example, in some particularembodiments, the high energy excitation radiation is provided by a modelocked femto second or pico second laser 501, e.g., a Ti:sapphire laseremitting 160 femto second pulses at a repetition rate of 76 MHz andhaving a wavelength of about 825 nm±30 nm. In some cases, it may bedesirable to reduce the intensity of the excitation light 586 that isincident on the NFT subassembly under test. In these cases a beamsampler may be used to pass a portion of the excitation light to a beamdump. As shown in FIG. 5, the excitation light 586 emitted by the laser501 reflects from an optional Fresnel beam sampler 502. A portion 586 bof the excitation radiation is transmitted through the Fresnel sampler502 to a beam dump 503. Another portion 586 a of the excitationradiation is directed toward an optional beam expander 504 that expandsthe beam of the excitation radiation 586 a emitted by the laser. In somecases, the system 500 optionally includes a subsystem 506 configured tomonitor the excitation radiation 586 a at the output of the beamexpander 504. The optional excitation radiation monitor 506, caninclude, for example, a neutral, non-polarizing beam splitter cube 506 athat splits off a sample 586 c of the excitation radiation 586 a anddirects the sample radiation 586 c to a photodetector 506 b. Thephotodetector 506 b generates a signal 506 c in response to the incidentsample radiation 586 c.

The excitation radiation 586 d passes through an achromatic halfwavelength wave plate 507 that rotates the polarization direction ofradiation 586 d to the desired direction for NFT excitation. Theexcitation radiation 586 d is focused by focusing lens 508 onto theinput waveguide coupler (or grating coupler) in subassembly 511 beingtested by end-fire technique. For example, a suitable lens for lens 508is an aspherical lens that has a numerical aperture (NA) of about 0.25.In some test setups, the NFT subassembly 511 being tested is disposed ona bar 514 that includes many NFT subassemblies. In response to theexcitation radiation 586 d, the NFT subassembly 511 being tested emitsphotoluminescent radiation and also a portion of the excitationradiation is transmitted through the NFT subassembly 511. Thus, theradiation 587 emanating from the NFT subassembly 511 is a combination ofthe photoluminescent radiation and the excitation radiation, aspreviously discussed.

The combined radiation 587 output from the NFT subassembly 511 undertest is collimated and collected by a lens 518 of high numericalaperture, e.g., NA of about 0.90. To image the radiation exiting surfaceof the NFT subassembly 511, the system 500 may include an imagingsubsystem 560. The imaging subsystem includes a fiber bundle white lightsource 520 that provides white light 521 for imaging the NFT subassembly511. The white light 521 is coupled into the light beam 588 by abroadband mirror 522. Arrow 525 indicates that components of the imagingsubsystem, e.g., the white light source 521 and mirror 522 may be usedfor set up and then removed from the beam path. Radiation 588 includesexcitation radiation transmitted through the NFT subassembly 511,includes photoluminescent radiation emitted by the NFT subassembly inresponse to the excitation radiation. In some configurations, theimaging system 560 is used to position the NFT subassembly 511. In theseconfigurations, the white light 521 generated by the white light source520 will not be a component of radiation 588 when the photoluminescenceof the NFT subassembly 511 is being measured.

Radiation 588 is optionally redirected through mirror 527 and through ashortwave pass spectral filter 530. The shortwave pass filter 530substantially blocks (absorbs) the excitation radiation andsubstantially passes a portion of the photoluminescent radiation emittedby the NFT. FIG. 6 shows a possible spectral characteristic for thefilter 530. A filter having the characteristics of FIG. 6 has an opticaldensity of 10⁻⁷ and blocks transmission by a factor of about 10⁻⁷ in thewavelength range longer than the cut-off wavelength (which is 650 nm inFIG. 7), where the excitation radiation (e.g., 825 nm+30 nm) is located.In contrast, the filter shown in FIG. 6 substantially passes radiation(has an optical density close to 0 or 100% transmission) in thewavelength range from about 320 nm to about 650 nm. Arrow 531 indicatesthat the filter 530 may be moved out of the radiation path duringmeasurement of the transmitted excitation radiation.

After the filter 530, a confocal detection scheme is used. Radiation 589that passes through the filter 530 subsequently passes through anon-coated or broad-band coated plano-convex imaging lens 535 or adoublet. An iris diaphragm or a slit 540 is placed near the focal pointof the imaging lens 535 to reduce the background noise. Radiation 590that passes through the iris diaphragm or slit 540 is imaged by abiconvex lens 545 and through a dichroic beam splitter 547. The dichroicbeam splitter 547 has a 685 nm edge that separates the incomingradiation 591 into two spectrally distinct beams. Any radiation withwavelength above the 685 nm edge is transmitted, whereas radiation withwavelength below the 685 nm edge is reflected. The spectralcharacteristic of a suitable dichroic beam splitter is shown in FIG. 7.The incoming radiation 591 is separated by the dichroic beam splitter547 into a first radiation beam 592 with wavelength greater than 685 nmand a second radiation beam 593 with wavelength less than 685 nm. Theshortwave pass filter 530 in combination with the dichroic beam splitter547 reject the excitation radiation wavelengths from the secondradiation beam 593 by a factor of about 10⁻⁷, or about 10⁻¹⁰ or evenabout 10⁻¹⁴.

The first radiation beam 592 is directed to a photodetector 550configured to measure the excitation radiation transmitted through anNFT subassembly. The second radiation beam 593 comprises the componentsfrom the photoluminescence from the NFT under test 511 and the lightfrom the white light source 520 that was reflected by the NFT bar 514.Optical element 555 directs the photoluminescent radiation to both oreither of PMT 570 and CCD 556. Optical element 555 is on a translationstage and may be a beam splitter or moveable mirror. If optical element555 is a moveable mirror, the moveable mirror directs the luminescenceto PMT 570 or to CCD 556. If optical element 555 is a beam splitter,optical element directs the luminescence to both the PMT 570 and the CCD556. With the white-light moved in the light path and without thepresence of shortwave pass filter 530 in the light path, the lighttransmitted through a device, including the NFT radiation, and the whitelight 594 reflected from a device is imaged onto a cooled charge coupleddevice (CCD) that is a part of the imaging subsystem 560; with thewhite-light moved out of the light path and the shortwave pass filter530 moved into the light path, the two-photon induced photoluminencefrom NFT is either directed to PMT 570 or CCD 556 if 555 is a mirror,or, is split into both PMT 570 and CCD 556 if 555 is a beam splitter.The photoluminescence image of the NFT bar 514 can be viewed using theCCD without the presence of white light radiation and with the presenceof shortwave pass filter. The white-light source is removed from theoptical path to measure photoluminescence (imaging and detection).

The photoluminescence 595 is detected by photomultiplier tube (PMT) 570.In response to the photoluminescence 595, the PMT 570 generates anelectrical signal 575 that is based on the amount of radiation incidenton the PMT 570. The electrical signal 575 is amplified using alock-amplifier that is locked to the repetition frequency of the laserpulses.

Some embodiments are directed to methods for characterizing NFTsubassemblies. With reference now to FIG. 8, a method to characterize anNFT subassembly includes providing 810 excitation to the NFTsubassembly. The excitation radiation generates photoluminescence in theNFT at a shorter wavelength than the excitation radiation. The radiationoutput from the NFT includes both the excitation radiation and thephotoluminescent radiation. The excitation radiation is substantiallyseparated 820 from the photoluminescent radiation, e.g., using ashortwave pass filter and/or dichroic beam splitter. Thephotoluminescent radiation is detected 830 by a detector that generates840 a signal indicative of the amount of photoluminescent radiation. Theamount of photoluminescent radiation is related to the functionality ofthe NFT and the generated signal can be used to determine how well aparticular NFT subassembly is working and/or to compare the relativequality of NFT subassemblies on a bar.

FIGS. 9A, 9C and 9D show optical images of the transmitted light from agap-plasmon NFT acquired using the system shown in FIG. 5. In FIG. 9A,the CCD shows the transmission image dominated by the excitationradiation. In FIG. 9A, the shortwave pass filter is not inserted in thelight path so that all the light, including the transmitted light andradiation emanating from the NFT, is observed (both photoluminescentradiation and the transmitted excitation radiation, which dominates).For the image of FIG. 9A, the CCD gain=1 and the exposure time is <1/500 second. For comparison, FIG. 9B shows the photoluminescentradiation for a device that does not have an NFT. As would be expected,the photoluminescent radiation for a device without an NFT is very weakand cannot be detected in FIG. 9B. In FIG. 9B, the shortwave pass filteris inserted in the beam. For the image of FIG. 9B, the CCD gain=4, theexposure time is 2 seconds, and the incident (average) power=3.6 mW).Incident power is the average power onto a device, i.e., the intensityof radiation 586 d in FIG. 5.

FIGS. 9C and 9D show the photoluminescent radiation from devices withNFT. The shortwave pass filter is inserted in the light path and thewhite-light source is removed out of the light path. In each of FIGS. 9Cand 9D, the photoluminescent radiation creates one bright optical spot.By comparing FIGS. 9C and 9D to FIG. 9B, it is apparent that thepresence of the NFT greatly enhances the photoluminescent radiationemerging from the device under test. For FIGS. 9C and 9D, the CCDgain=4, the exposure time=2 seconds, and the incident (average) power isreduced to only 0.8 mW.

FIG. 10 shows a log-log plot of photoluminescence vs. monitor power,obtained using the NFT characterization system described in FIG. 5 fordevices that have a lollipop NFT. The fit of luminescence with incidentpower of 2.2 is observed, which is a signature of two-photonluminescence.

FIG. 11 shows the variation in photoluminescence for lollipop NFTsmeasured using the NFT characterization system shown in FIG. 5. A bar of64 NFT subassemblies with lollipop NFTs was tested using thecharacterization approaches discussed herein. In this example, thephotoluminescence varied across the bar.

It is to be understood that even though numerous characteristics ofvarious embodiments have been set forth in the foregoing description,together with details of the structure and function of variousembodiments, this detailed description is illustrative only, and changesmay be made in detail, especially in matters of structure andarrangements of parts illustrated by the various embodiments to the fullextent indicated by the broad general meaning of the terms in which theappended claims are expressed.

What is claimed is:
 1. A system comprising: an excitation light sourceconfigured to provide excitation radiation to a near field transducer(NFT) subassembly including an NFT optical antenna; an optical filterconfigured to substantially pass a portion of photoluminescent radiationemitted by the NFT optical antenna in response to the excitationradiation and to substantially block the excitation radiation; and adetector configured to detect the portion of photoluminescent radiationand to output an output signal in response to detection of the portionof photoluminescent radiation.
 2. The system of claim 1, furthercomprising: a focusing lens configured to focus the excitation radiationonto an input coupler of the NFT subassembly; and an objective lensconfigured to collect the photoluminescent radiation emitted by the NFToptical antenna toward the optical filter;
 3. The system of claim 1,wherein the excitation radiation comprises femto-second or pico secondlaser pulses.
 4. The system if claim 1, wherein the optical filter isconfigured to substantially pass radiation having wavelengths shorterthan the excitation light.
 5. The system of claim 1, further comprisinga monitoring subsystem configured to monitor the excitation radiation.6. The system of claim 5, wherein the monitoring subsystem comprises: aninput side photodetector configured to generate a signal in response tothe excitation radiation incident on the input side photodetector; abeam splitter disposed in between the laser and the NFT subassembly, thebeam splitter configured to split the excitation radiation so that afirst portion of the excitation radiation travels along a first inputpath toward the input side photodetector and a second portion of theexcitation radiation travels along a second input path toward the NFTsubassembly.
 7. The system of claim 5, wherein the monitoring subsystemcomprises an output side photodetector configured to generate a signalin response to output radiation transmitted through the NFT subassembly,the output radiation dominated by the excitation radiation.
 8. Thesystem of claim 7, further comprising a beam splitter disposed betweenthe NFT and the output side photodetector, the beam splitter configuredto split the output radiation so that a first portion of the outputradiation having wavelengths longer than an edge wavelength of the beamsplitter travels along a first output path toward the output sidephotodetector and a second portion of the output radiation havingwavelengths shorter than the edge wavelength travels along a secondoutput path toward the detector.
 9. The system of claim 1, furthercomprising an imaging subsystem configured to provide a visual image ofthe NFT.
 10. The system of claim 1, wherein the imaging subsystemcomprises: a white light source; and a charge coupled device (CCD). 11.The system of claim 1, wherein the detector comprises a photomultipliertube (PMT).
 12. The system of claim 1, further comprising an amplifiercoupled to receive the detector signal, wherein the amplifier islocked-in to a repetition frequency of the excitation radiation.
 13. Amethod, comprising: providing excitation radiation to a near fieldtransducer (NFT) subassembly; filtering output radiation from the NFTsubassembly, the filtering comprising passing a portion ofphotoluminescent radiation emitted by the NFT subassembly in response tothe excitation radiation and substantially blocking the excitationradiation transmitted by the NFT subassembly; and detecting the portionof photoluminescent radiation and generating an output signal inresponse to the detecting.
 14. The method of claim 13, wherein filteringthe output radiation comprises filtering using a shortwave pass opticalfilter.
 15. The method of claim 13, wherein filtering the outputradiation comprises filtering using a dichroic beam splitter.
 16. Themethod of claim 13, wherein detecting the portion of photoluminescentradiation comprises detecting using a photomultiplier tube.
 17. Themethod of claim 13, wherein providing excitation radiation to an NFTsubassembly comprises providing laser pulses at a repetition rate. 18.The method of claim 13, wherein filtering the output radiation from theNFT subassembly comprises substantially blocking the excitationradiation and substantially passing the portion of the photoluminescentradiation.
 19. The method of claim 18, wherein blocking the excitationradiation comprises blocking the excitation radiation by a factor ofabout 10⁻⁷.
 20. The method of claim 13, further comprising signalprocessing the output signal including amplifying the output signalusing circuitry that locks into a repetition rate of the excitationradiation.